1. Field of the Invention
The present invention generally relates to a plasma-process system, and more particularly to a plasma-process system including a plasma-process method with improved end-point detecting scheme and also a plasma-process apparatus with improved end-point detecting scheme.
2. Description of the Related Art
In the manufacture of semiconductor devices, dry etching is a process indispensable to the forming of fine IC patterns on a semiconductor wafer. Various types of dry etching are known, among which is plasma process. In the plasma process, plasma is generated from reactant gases within a vacuum chamber, and ions, neutral radicals, atoms, and molecules--all present in the plasma--are applied to a semiconductor wafer, thereby removing things to be removed, such as a silicon dioxide film in non-masked potions on the wafer.
If the plasma process, or plasma etching, is continued even after the things to be removed which should be removed, have been completely removed from the semiconductor wafer, the surface of the wafer will be unnecessarily etched away, or the resist layer on the wafer will be etched to become thinner than is required. Hence, it is required that the end point of the plasma etching be detected with high accuracy.
In a typical conventional method of detecting the end point of plasma etching, the luminous intensity of the product of plasma etching is monitored, and the end point of the plasma etching is determined from the changes in the luminous intensity. In the case of plasma etching, wherein a silicon dioxide film is etched with CF-based reactant gas, the luminous intensity of carbon monoxide, i.e., the reaction product, is monitored, and the end point of plasma etching is determined from the changes in the luminous intensity of the carbon monoxide. (Refer to Published Unexamined Japanese Patent Application No. 63-81929 and Published Unexamined Japanese Patent Application No. 1-230236.)
More specifically, the plasma etching is performed in a reaction chamber containing plasma electrodes and having an observation window. The observation window is made of quartz glass and has a diameter of several centimeters. An optical system including a lens is fixed outside the reaction chamber and located near the observation window. The light emitted from the plasma during the plasma etching is applied through the window to the optical system. The optical system collects the light and outputs it to a photoelectric transducer or the like through an optical fiber. The transducer converts the input light into electric signals, which are supplied to a detecting section. The detecting section processes the signals, thereby detecting the end point of the plasma etching. In the case of plasma etching for etching a silicon dioxide film with CF-based reactant gas, the light emitted from the carbon monoxide (i.e., the reaction product) is collected by an optical system and converted into electric signals by a photoelectric transducer, and these signals are processed to monitor the changes in the luminous intensity of the carbon monoxide and ultimately to determine the end point of the plasma etching.
The end point of the plasma etching is determined for the following reason. As long as the plasma etching proceeds, the reaction product is continuously formed. When the plasma etching comes to an end, that is, when the thing to be etched is etched away completely, the forming of the reaction product stops, whereby the luminous intensity of the product decreases abruptly. Hence, the end point of the plasma etching can be detected by finding an abrupt decrease in the luminous intensity of the reaction product, at a specific wavelength.
For a successful plasma etching, it is necessary to stabilize the plasma in the reaction chamber or to increase the efficiency of etching an undercoating film or a resist layer. To this end, a gas is used in addition to the etching gas, in an amount greater than the etching gas. For example, to stabilize the plasma, argon gas is added to CF-base gas. In this case, the emission spectrum of the carbon monoxide (i.e., the reaction product) overlaps that of the argon gas, which is very broad.
This overlapping of spectra is inevitable for two reasons. First, the wavelength of the intense light the argon gas emits and that of the intense light the carbon monoxide emits fall in a range of 300 to 800 nm; in other words, the light beams emitted from the argon gas and the carbon monoxide have very similar wavelengths. Secondly, the carbon monoxide is generated in a very small amount since that the etching reaction is proceeding at only a small portion of the wafer, whereas the argon gas is introduced into the reaction chamber in an amount several to ten times greater than the etching gas applied. As a result, the luminous intensity of the carbon gas is far greater than that of the carbon monoxide.
Since the emission spectrum of the carbon monoxide overlaps that of the argon gas, for the reasons mentioned above, the luminous intensity of the carbon monoxide can hardly be found correctly. Consequently, it is difficult to detect the end point of the plasma etching in the conventional method.
In the conventional method, the luminous intensity of the reaction product is monitored over a wavelength range of about 300 to 900 nm, because of the limited sensitivity of the luminous intensity detector used, such as a spectrometer. In particular, the luminous intensity is monitored with respect to a wavelength of 482.0 nm at which the peak intensity is found in the emission spectrum of carbon monoxide. Obviously, the wavelength range of 300 to 900 nm is very similar to the range of 350 to 860 nm for the intense light which argon gas emits. Consequently, the emission spectrum of carbon monoxide overlaps that of the plasma itself, making it difficult to detect the luminous intensity of carbon monoxide with sufficiently high accuracy.
In recent years it has been demanded that finer IC patterns be formed on a semiconductor wafer. To meet this demand, the IC patterns formed on wafers at present are actually finer than those formed before. Thus, the tendency is that the ratio of the etched surface portion to the entire surface area of a semiconductor wafer, i.e., the rate of hole area, is now 10% or less. In some cases, this ratio is as small as 2% or 3%. The amount of the carbon monoxide, generated generated during plasma etching, is extremely small, for example only 1% or less of the amount of the carbon gas introduced into the reaction chamber. Hence, the conventional method can no longer serve to detect the luminous intensity of carbon monoxide with high accuracy, making it difficult to determine the end point of the plasma etching.
Further, to meet the demand for the forming of finer IC patterns, or the increasing demand for the forming contact holes only, trenches only or wiring patterns only by means of plasma etching, the rate of hole area is likely to be only 1% or even less at present. As a consequence, the change in the amount of etching gas supplied immediately before or after the end of plasma etching is extremely small. Hence, the intensity of the emission spectrum of the etching gas changes very little shortly before or after the end of plasma etching, and the luminous intensity of the gas is extremely low shortly before or after the end of plasma etching. Thus, with the conventional method it would be impossible to detect the end point of plasma etching with high accuracy.
In another conventional method, not only the luminous intensity of the reaction product, but also the luminous intensity of the etching gas is monitored, and the end point of plasma etching is determined from the difference between the luminous intensity of the product and that of the gas or from the ratio of the former to the latter. This method is based on the assumption that the amount of the etching gas in the reaction chamber remains almost constant during the plasma etching, but increases at the end thereof.
This conventional method, however, is disadvantageous, too. The amount of the etching gas and that of the reaction product do not always remain constant throughout the etching. In some cases, the luminous intensity of the gas and that of the product decrease as the etching proceeds. This is perhaps because of the changes in the operating conditions of the exhaust system, the changes in the temperature in the reaction chamber, or the like. Consequently, the end point of the plasma etching cannot be correctly determined, solely from the difference between the luminous intensity of the product and that of the gas or the ratio of the luminous intensity of the product to that of the etching gas.
Particularly, in the case where the luminous intensity of an etching gas gradually decreases during the plasma etching proceeds, more than it changes at the very end of the etching, it is difficult to distinguish the change during the etching from the change at the end of the etching, or vice versa. It would be more difficult to do so in the case where the rate of hole area is low, since the luminous intensity of the etching gas changes only a little at the end of the plasma etching.
As has been described, the optical system is fixed outside the reaction chamber in a prior-art plasma-process apparatus. Here arises a problem. The position where the plasma is generated changes in accordance with the size of the semiconductor wafer being plasma-processed and also with the conditions of plasma process. Hence, in the prior-art plasma-process apparatus, it is impossible to detect the peak point of the plasma by means of the optical system, which cannot be moved at all.
In addition, the plasma is generated within the reaction chamber in various manners, depending on the conditions of the plasma process such as the size of the wafer, the way of processing the wafer, and the composition of the etching gas used. As a result of this, the peak point of the plasma, i.e., the point where the plasma has the highest luminous intensity, moves back or forth, and up or down, in the space between the plasma electrodes, every time the plasma-process conditions are changed.
With the prior-art plasma-process apparatus, wherein the optical system containing a lens used for detecting the luminous intensity of plasma is fixed in place, the peak point of the plasma can not always located. Hence, the apparatus cannot detect slight changes in the luminous intensity of the plasma, making it even more difficult to detect the end point of plasma etching. In view of this, the conventional plasma-process apparatus needs some improvement.
The recent tendency in the art is that a light beam having a short wavelength, e.g., 200 nm, which is shorter than any one within the emission spectrum of argon gas, is observed so that its luminous intensity is detected. However, as much as 50% of the light having such a short wavelength is absorbed, in some cases, by the optical fiber as it is applied from the optical system to the photoelectric transducer or the like.
Furthermore, the reaction product formed during the plasma-process sticks onto the inner surface of the observation window of the reaction chamber, inevitably rendering the window opaque. The amount of light applied from the window to the optical system is less than otherwise. This makes it still difficult to determine the end point of plasma etching with accuracy, since it is necessary at present to detect the end point from the changes in the plasma luminous intensity which are extremely small in most cases for the aforementioned reasons.